Jacob H. has already answered this in relation to a version of Eudora,
but for those using UNIX Pine with the Pico editor, who encounter
similar problems, this:

When you do a GET of whatever and you know that if your line breaks
in the wrong place due to your editing, you can put it back together
with the longer line.  Do your editing as needed, don't worry 'bout the
breaks.  When you are finished, ready to send it off in a form LISTSERV
will accept, go back to the start of the line that is really the tag
of the previous and do a "backspace" paying due attention to whether you
have a "space" there as needed or not; voila, the edited line Pine/Pico
thinks is too long is now ready to send in the form LISTSERV requires.

As good ole Pete might say, YMMV.     Douglas